Pathological protein aggregation is ubiquitously associated with neurodegenerative disease, but the underlying mechanisms are largely unclear. Recent groundbreaking work suggests a role for intracellular liquid phase transitions in governing the assembly and organization of functional protein assemblies or granules with liquid-like properties. This proposal seeks to test the hypothesis that misregulation of intracellular liquid phase separation leads to detrimental protein aggregation. In the mentored phase, Aim 1 seeks to define the physicochemical parameters and phase landscape of stress granule proteins implicated in disease. This aim will be achieved through applying principles of engineering and soft matter physics along with unique rheological methodologies previously developed by Dr. Elbaum. Aim 2 seeks to determine the relationship between liquid phases, aggregation and toxicity in vivo as a function of aging and stress. Using the C. elegans model system, this aim will establish whether liquid protein phases prevent or promote amyloid formation, and garner insight into whether aging and/or stress contribute to regulation of protein assembly. In the independent phase, Aim 3 seeks to identify molecular pathways regulating protein assembly and further resolve the molecular mechanisms underlying regulation. This aim will be achieved through a unique multi-scale approach combining single molecule techniques, material science methodologies, and organismal level interrogation. Together, this innovative strategy will offer novel insight into the biogenesis of pathological protein aggregation, and define new pathways and distinct molecular targets for therapeutic interventions aimed at preventing or reducing the health burdens of neurodegenerative disease.